1. Field of the Invention
The present invention relates to a novel process for impregnating a fibrous, filamentary and/or porous network with powder, especially in order to produce a composite comprising a continuous, rigid or flexible, matrix with which said network is in intimate contact.
2. Description of the Related Art
Composites reinforced by fibers embedded in thermoplastic or thermosetting matrices are an extremely interesting class of materials, allowing in particular the production of materials exhibiting excellent mechanical properties for masses substantially less than those of metals. Furthermore, these materials are obtained by simple molding, after having coated the reinforcing fibers or filaments with the resin intended to form the matrix of the composite. Of course, the mechanical properties of the composite thus obtained depend on the quality of the interface between the reinforcing fibers or filaments and the matrix.
This therefore assumes that there is good cohesion between the fibers or filaments and the matrix. Two factors essentially determine this cohesion: these are, on the one hand, the adhesion properties between the resin and the reinforcing fibers or filaments, that is to say the selection of the material intended to form the matrix, and, on the other hand, the void fraction inside the composite. This second factor results of course from the ability of the resin to infiltrate between the fibers and the filaments of the fibrous mass. This is because each fiber or filament or each fiber or filament portion that is not coated with the matrix does not contribute or contributes only partly to the mechanical properties of the composite. Consequently, the higher the void content the lower the mechanical properties of the composite.
Fibrous materials used for acoustic or thermal insulation are produced by bonding the fibers constituting a nonwoven at the point of intersection between the fibers with an adhesive or hot-melt adhesive. These materials may be produced by passing the nonwoven through a bath of adhesive or by using hot-melt adhesive fibers or powders. The difficulty in the case of hot-melt adhesive powders is that it requires the use of a process allowing the powders to be distributed at the points of intersection of the fibers, in order to optimize bonding, while limiting the amount of powder used.
Functional textile materials require the introduction of an active (bactericidal, fireproofing, superabsorbent, etc.) principle into the base textile. This active principle may be in the form of liquid solutions or in the form of dry powders with which the textile substrate will have to be impregnated. The case of active principles in liquid form has the drawback of requiring a substantial consumption of energy in order to be able to dry them, while evaporating the solvent from the solution. Powders do not have this type of problem, but they are sometimes difficult to distribute uniformly and definitively within the base textile.
WO 99/22920 discloses a process for impregnating powder into a fibrous or filamentary network, characterized in that the powder on the one hand and said network of fibers or filaments on the other are placed in an electric field whose AC voltage is at least 5 kV for a time of at least 2 s. That document discloses, as electrodes, two superposed parallel metal plates connected to the two respective poles of an electric generator, the respective faces of which, facing each other, are covered with a dielectric plate, for example a glass ceramic plate.
That process has many advantages, but it is ill-suited to the impregnation treatment of large articles, for example those from 0.80 to 7.00 m in width, in particular when the continuous treatment is carried out with a material running between the electrodes. This is because any geometrical distortion of the electrodes results in variations in the distance between them, which degrades the uniformity of the electric field and therefore the quality of powder impregnation. To avoid this geometrical distortion, the lower metal plate electrode may be placed on a stand; however, the upper metal plate electrode and the dielectric that covers it therefore have to be stiffened, for example by transverse bars, which may impair the operation of the electrodes.